Internal combustion engines generate power by igniting a mixture of fuel and air. In order to get the most power from the fuel, that is, for the most efficient operation, many parameters of the combustion process must be optimized. Much research has gone into finding ways to increase efficiency by varying these parameters. As a result, the fuel efficiency, and hence fuel economy of vehicles, has improved dramatically.
Two of the important factors affecting the efficiency of internal combustion engines are air/fuel ratio and volumetric efficiency. Volumetric efficiency is defined as the volume flow rate of air into the intake system divided by the rate at which volume is displaced by the piston. Not only is it important for these parameters to be optimized for each cylinder but it is also important to minimize variations in these parameters. Such variations can occur within a given cylinder at different times, or can occur between individual cylinders in an engine. Undesirable variations in combustion parameters will create imbalances in the engine's operation that will affect efficiency and overall performance.
One example of such an imbalance is known as "odd fire" power impulses. This occurs, for example, in two cylinder, 45-degree V-Twin motorcycle engines. In these engines there occurs an interval of 315 degrees of crankshaft rotation between a first pair of power impulses, followed by an interval of 405 degrees between the following two power impulses. The different spacing results in different air/fuel conditions occurring in the intake manifold when the charge enters the cylinder. Such differences may result from variations in factors such as temperature, pressure or turbulence inside the intake manifold.
In any event, these variations cause the cylinders to fire with different air/fuel ratios and different volumetric efficiencies depending on whether they fire after the 315-degree interval or the 405-degree interval. This affects the way the engine operates, for example it results in timing differences in back-pulse waves through the induction system before mechanical induction demand. This can adversely affect the performance of the engine, causing it to run less smoothly and with lower efficiency. Thus, it would be desirable to provide a way to minimize the variations in the air/fuel conditions between cylinders that result from "odd fire" power impulses.